A standard metallized film capacitor widely known in the art is the wound capacitor. Wound capacitors are constructed by sandwiching a dielectric film such as polycarbonate, polypropylene or polyester film between metal electrodes (e.g., vapor deposited metal film). Once formed, the combination dielectric/metal material is wound to form a capacitor. Some specific examples of wound capacitors are found in the following: U.S. Pat. No. 4,320,437 (Shaw et al.), U.S. Pat. No. 4,719,539 (Lavene), and U.S. Pat. No. 4,685,026 (Lavene).
The maximum electrostatic energy that can be stored in a metallized film capacitor depends on the total capacitance of the capacitor and the square of the maximum voltage that can be safely applied across the capacitor (its breakdown voltage). The breakdown voltage of a capacitor depends on the dielectric strength and the thickness of the film.
Related to the breakdown voltage is the number of shots a capacitor, when used as an energy storage device, can withstand. A "shot" is the two step process of (1) charging the capacitor and, then, (2) discharging the stored energy, in the form of a pulse, into a low impedance load (e.g., (1) a human body is approximately a 40 ohm load and (2) a strobe is approximately a 4 ohm load). For general applications, capacitors are rated such that they can withstand on the order of 100,000 shots; however, special applications may only require a limited number of shots.
Also related to the breakdown voltage is the size of the capacitor. The size of a metallized film capacitor is substantially dictated by the thickness of its dielectric film. The thickness of the dielectric, in turn, is dictated by the required overall breakdown voltage of the capacitor. For instance, if a manufacturer cites a particular film as having a dielectric strength of 200 volts/.mu. and the capacitor design calls for a dielectric breakdown voltage of 400 volts, then the film may be 2.mu. thick.
It should be noted that the thickness of the metallization (typically referred to in ohms/sq), although it contributes to the size, is not as substantial as that of the dielectric. Typical metallization thickness range from approximately 1-4 ohms/sq.
Electrolytic capacitors have been commonly used as energy storage devices because they can be made small with high energy storage capability. However, electrolytic capacitors have many drawbacks. The drawbacks include: (1) a high dissipation factor, (2) capacitance decreases with increasing frequency, (3) capacitance substantially decreases with decreasing temperature, (4) because electrolytic capacitors are very lossy, they produce only about 80% efficiency on discharges, (5) electrolytic capacitors tend to leak and (6) if electrolytic capacitors remain idle for an extended period of time, the oxide on the aluminum must be reformed which requires precious battery power.
A particular application for which electrolytic capacitors have been used instead of metallized film capacitors, primarily because of size requirements, is in implantable defibrillators. In a recent IEEE Spectrum article, however, discussing implantable defibrillators, it was stated that "[f]uture generations of defibrillators are likely to be smaller. Today's models are about the size of a bar of soap, and shrinking them further will require new kinds of batteries and capacitors. Defibrillator manufacturers, who currently use aluminum electrolytic photoflash capacitors, are working on custom capacitors that will help reduce implant size." IEEE Spectrum, "Technology 1993", January 1993, pg. 76, col. 3.
In an implantable defibrillator, the capacitor is used as an energy storage device. A battery is used to charge the capacitor which, in turn, delivers a shot to the patient's heart in order to correct, for instance, ventricular tachycardia (ventricles beating to rapidly) or ventricular fibrillation (ventricle quiver chaotically). In an implantable defibrillator, the capacitor need only be capable of delivering from 3 to 6 shots. Practically speaking, if the defibrillator is unable to correct the problem within 6 shots, it is unlikely that the patient will survive, hence, eliminating the need for additional shots.
Thus, it would be advantageous, particularly for applications such as implantable defibrillators, to have metallized film capacitors which match, or even better, the size of comparable electrolytic capacitors.